You are definitely talking about different thing.

You are talking about that a lens (say 24mm lens), and its center section (ie, 1/1.6 portion) has less vignetting than the part outside the center 1/1.6 portion. Of course that is true (vignetting, by definition, is a dimming on the edges and corners, not the center). But the two photos will have different angles of view (the one from a crop camera will not qualify as an ULTRA WIDE ANGEL PHOTO).

I am talking about taking an ultra angle photo (say 100 degrees of view). I do not care how you achieve it (using a crop camera or a full frame camera). What I am saying is with a crop camera (plus the appropriate lens), the vignetting is not going to be less than that on 5D (with appropriate lens).